This invention relates generally to nuclear fusion reactors of the tokamak type and is particularly directed to a neutral beam shield and plasma limiter for use in a tokamak fusion reactor.
Among the various approaches currently under evaluation as a potential long term source of energy produced by nuclear fusion is the magnetic confinement of an energetic plasma in the form of a toroid or "doughnut". The apparatus used in the confinement, excitation and extraction of energy from the plasma is known as a tokamak fusion reactor. A tokamak fusion reactor includes a circular arrangement of powerful magnets for generating a toroidal magnetic field wherein an energetic plasma comprised primarily of protons, deuterons, tritons and electrons is confined. The toroidally confined plasma may be energized by various means including the injection of energetic neutral particles, typically deuterium, not influenced by the magnetic field which are able to penetrate into and heat the plasma. The thermal energy thus produced within the plasma causes the nuclei therein to fuse with the release of substantial energy.
The confining magnetic field of a tokamak reactor is generally toroidal with the magnetic field lines made to spiral in the toroidal direction by a poloidal field produced by current flowing within the plasma. This plasma current can also be used to produce ohmic heating of the plasma which is generally supplemented by means of the aforementioned neutral beam injection of energetic particles to attain those temperatures necessary for the fusion of nuclei. Supplementing of the ohmic heating of the plasma with energetic neutral particles is necessary primarily due to a decrease in the resistance of the plasma with increasing temperatures.
Although the plasma of a tokamak reactor is confined by a magnetic field, the vacuum chamber within which the plasma is generated and confined typically includes various structures for defining the size and shape of the confined plasma. These structures are generally referred to as plasma limiters, or limiter blades, and they may either be fixed or movable within the vacuum chamber.
Whether stationary or movable, a plasma limiter must be capable of withstanding not only tremedous thermal loads, but also extremely large electromagnetic forces and mechanical vibrations arising from the pulsed nature in which the plasma is energized. In addition, substantial mechanical loading of the plasma limiter may result from plasma disruptions as eddy currents induced in the plasma limiter react with the rapidly changing magnetic field. These plasma limiters not only function to form the plasma in a desired shape, but also serve to shield various components within the tokamak reactor from the hostile environment within the plasma chamber. For example, various electromagnetic and thermal sensors are provided around the periphery of the plasma chamber in order to monitor the various parameters of the environment therein representing the behavior and characteristics of the heated plasma. In performing this function, the limiter must provide protection for a given sensor, while orienting and positioning the sensor for maximum sensitivity. The plasma limiter must also provide shielding for the various coils producing the confining magnetic field while allowing for their proper positioning relative to the toroidal vacuum chamber.
The present invention provides all of the aforementioned features in a toroidal midplane neutral beam armor and plasma limiter which is positioned on the inner wall of a tokamak fusion reactor for shielding a a midplane magnetic coil while allowing a magnetic flux loop sensor to be positioned in close proximity to the heated plasma. The armor shield/plasma limiter of the present invention is uniquely adapted to withstand the extreme thermal loads, electromagnetic forces, and mechanical stress present in a pulsed tokamak fusion reactor while providing the required plasma control and an ultra clean environment within the plasma chamber.